The invention relates to a multilayer membrane, which is composed of a carrier layer, a porous supporting layer and a separating layer, and to a process of manufacturing the same.
European Patent Specification EP 96 339 B1 has already disclosed a composite membrane comprising a nonporous separating layer made of a first polymer and a porous supporting layer made of a second polymer, wherein the nonporous separating layer consists of cross-linked polyvinyl alcohol or cellulose triacetate and the polymer used for the separating layer of the membrane has not entered the pores of the supporting layer. It is also known from that European patent specification that the porous supporting layer has been applied to a nonwoven fabric or a woven cloth used as a carrier layer and that the porous supporting layer consists of polyacrylonitrile or polysulfone. It has also been proposed in that European patent specification that the cross-linked separating layer made of polyvinyl alcohol has been cross-linked by etherification, esterification or acetalization or by a combination of said processes and has been rendered water-insoluble by an action of heat. The laminated membrane known from the above-mentioned European Patent Specification can desirably be used to separate liquid mixtures by pervaporation. But it has been found that the separating capacity of the known laminated membrane--i.e. the rate at which the mixed material is separated into its constituents per square meter of the membrane surface area--is only low and that the separating capacities of individual membranes differ and vary during continuous operation.